Microfabrication with Very Low-Average Power of Green Light to Produce PDMS Microchips

Abstract

In this article, we show an alternative low-cost fabrication method to obtain poly(dimethyl siloxane) (PDMS) microfluidic devices. The proposed method allows the inscription of micron resolution channels on polystyrene (PS) surfaces, used as a mold for the wanted microchip's production, by applying a high absorption coating film on the PS surface to ablate it with a focused low-power visible laser. The method allows for obtaining micro-resolution channels at powers between 2 and 10 mW and can realize any two-dimensional polymeric devices. The effect of the main processing parameters on the channel's geometry is presented.

Keywords: PDMS devices; laser ablation; low-cost fabrication tool; microfabrication; polymeric microchip.

Figure 1

Fabrication method to obtain poly(dimethyl siloxane) (PDMS) devices with three main fabrication steps, (1) female mold (FM) fabrication, (2) male mold (MM) obtaining, and (3) chip assembling. (A) Layer film made on a face of polystyrene (PS) spectrophotometer cuvette by black sharpie marker (FML), (B) laser writing of the microchip features into the FML, (C and D) FM obtained by cleaning the ablated cuvette surface and cutting the imprinted side, (E and F) resin layer casting on the FM and peeled off to obtain a MM, (G and H) PDMS were cast on the MM and peeled off from it to get a polymeric chip layer, and (I) the final microchip is obtained by assembling the PDMS onto a glass slide.

Figure 2

The optical system imprints the microchip features on a surface of a polystyrene cuvette. A green laser beam with an initial profile is rectified by a circular Mask. Two mirrors (M1, M2) redirect the beam toward the FML supporting stage. A circular pinhole filters the central part of the beam from their diffracted components as a Pupil. Two linear polarizers (P1, P2) set a convenient beam polarization. A plano-convex lens (L) focuses the beam at the surface of the FML. Three linear motorized stages (X, Y, Z, Thorlabs MTS50) are used.

Figure 3

Micromachining by green laser ablation method. Two examples of FMLs, (a) and (b), obtained onto one side of the polystyrene spectrophotometer cuvette. (a) FML sample with different focal planes and (b) focused on a plane coincident with the ablated surface (Z = 0) (see Supplementary Information, Top view of sample and PDMS microchip Model 1).

Figure 4

FM sample is obtained onto one side of the polystyrene spectrophotometer cuvette of 10 × 18 mm², using a speed of 0.02 mm s⁻¹, and average power of 9.3 ± 0.1 mW, focused on a plane coincident with the ablated surface (Z = 0). The (a) shows a scanning electron microscope image of a top view of a polystyrene (PS) cuvette for conventional spectroscopic use. (b) Scanning cross-section image of an 18 mm long microchannel.

Figure 5

Laser micromachining channel width as a function of the beam average power for a beam focused on the ablated surface (Z = 0). The laser-induced damage threshold is about 2 mW. For a beam power smaller than this value, no ablation marking was observed.

Figure 6

Channel width (S) as a function of the focal plane distance to the ablated surface (Z) for different transversal (X or Y) speed displacements: $v_1=$0.02 mm s⁻¹ (blue circles),$v_2=$0.2 mm s⁻¹ (green circles), and $v_3=$2 mm s⁻¹ (black circles). Z > 0 for the beam focused within the ablated material, and Z < 0 for the beam focused before the FML surface. The Gaussian fitted beam diameter as a function of Z in Equation (1), in red, correlates well with the observed channel width for the explored displacement velocities for $\left|Z\right|<1 \mathrm{mm}$.

Figure 7

Channel depth as a function of different translation stage speed. The speed of the translation stages changes the depth of the channels’ range, from 2.8 ± 0.18 to 6.7 ± 0.2 μm, and the logarithmic approximation using Equation (2) (black line).

Figure 8

PDMS microchip device. The microchip consists of a poly(dimethyl siloxane) (PDMS) microfluidic device on the top of a glass substrate. This structure supports an alternative low-cost fabrication method with which a low-power visible ablation is possible. The insets show a schematic view of our microchip.